Variable-ratio electronic counter-divider

ABSTRACT

In a digital frequency dividers with a programmable ratio, there is provided two serially connected division scales, the first scale being set so that its division ratio shifts in whole units under the action of control signals applied thereto, the second scale including a fixed-capacity counter set to count permanently and to apply to the first scale a control signal which is enabled for a predetermined number of counts of the second scale. Such a divider is adapted for operation at frequencies higher than that of the prior art dividers, and will be used in frequency generators.

United States Patent 1191 Charbonnier 45 M 14, 1974 VARIABLE-RATIO ELECTRONIC 3.733.475 5/1973 Hueusler ..'328/39 COUNTERJMVIDER 3.283.254 11/1966 Haynie 328/48 3,137.818 6/1964 Clapper.. 328/43 Inventor: Roger Chflrbonnier Meudon, 3,675,049 7 1972 Haven......,... 307/223 France 3.280.309 10/1966 Villwock 307/223 {73] Assignee: Adret Electronique, Trappes. France Primary'Examiner-Rudolph V. Rolinec [22] Filed: 1972 Assistant Examiner-R. E. Hart [21] APPL 298,424 Attorney, Agent. or FirmWilliam Anthony Drucker [30] Foreign Application Priority Data [.57] ABSTRACT Oct. 18, 1971 France 71.37407 In a digital frequency dividers with a programmable ratio, there is provided two serially connected division [52] US. Cl 328/39, 328/43, 328/48 scales, the first scale being set so that its division ratio [51 Int. Cl. H03k 23/06 shifts in whole units under the action of control signals [58] Field of Search 328/39-58 applied thereto, the second scale including a fixedcapacity counter set to count permanently and to [56] References Cited apply to the first scale a control signal which is en- UNITED STATES PATENTS abled for a predetermined number of counts of the 3.581.116 5/1971 Leostic 328/41 Second 3.218.560 1 H1965 Peters 328/39 Such a divider is adapted for operation at frequencies 3.624.517 lI/l97l Kobayashi 328/44 higher than that of the prior art dividers, and will be 3.646.371 2/1972 F130 328/48 used in frequency genefators 3.426.180 2/1969 Smith 328/39 3.671.872 6/1972 Pauly 328/41 15 Claims, 12 Drawing Figures WENTEDHAY 14 1974 SHEET 1 [IF 4 FIG.2

FIG?) I I l I I T I l II VARIABLE-RATIO ELECTRONIC COUNTER-DIVIDER.

This invention relates to the variable capacity electronic counters used in frequency dividers.

In some applications of digital frequency dividers with aratio which can be programmed as a function of an instruction number, e.g. the phase servo-control loops which are comprised in some frequency generators, it would be useful to be able to make the variablecapacity counter which constitutes the basic element of these dividers work at higher frequencies than those which are currently made possible by known connections.

A known connection consists e.g. of associating with the counter a coincident circuit with a predetermined digital value N. At the Nth pulse, the coincident'circuit returns the counter to zero. The connections resolution time is then equal to the sum of the coincident circuits response time, to which must be added the counters return-to-zero time and its re-sensitization time. This resolution time is thus considerably increased at the moment of coincidence. Now, the maximum frequency that the divider can handle is the inverse of its working cycles longest resolution time.

To reduce this resolution time, various methods have been used and, notably, the one termed alternate operation which consists of associating, in-order to obtain a capacity N, two capacity counters, N-P and P respectively, the first counting N-P pulses, switching to the second counter then being effected until the latter has reached the counter P, at which moment a further switch is made to the first counter. Whenever they coincide, one of the counters is returned to zero, and during this return to zero, the other counter has time to be re-sensitized, which reduces the connections resolution time.

Nevertheless, this method does not enable the de vices maximum resolution time, which is governed by the sensing time of the counters N-P or P state and the switching times, to be reduced as much as one would wish.

The invention proposes the making of a divider wherein the resolution time is constant and reduced to the response time of a J K flip-flop, which makes it possible to work at considerably higher frequencies than those made possible by known connections. The frequency divider in accordance with the invention includes first and second division scales connected in series, and is chiefly characterised in that the first scale is set so that its division ratio shifts in whole values under the action of control signals, the second scale including a fixed-capacity counter set to count permanently and to transmit to the first scale a control signal which is enabled for a predetermined number of counts of the second scale. In accordance with a particular form of embodiment, the second scale moreover includes means of comparing the counters count with at least one instruction number to enable at least one control signal and also shiftthe division ratio by at least one unit.

In accordance with another peculiarity of the invention, the countercomprises a -N scale followed by a -two scale, and the comparator is set to compare the binary number expressed by the logical levels of the parallel outputs of the -N scale with a binary number n a/2, n being the instruction number and a a zero value when is even, and equal to I when n is odd, and the device comprises means, associated with the comparator, of enabling or otherwise the comparator output signal, according to the parity of n, said means only acting for N of the counters counts.

A better understanding of the invention will be obtained from the following description.

In the attached drawing:

FIG. 1 is a basic diagram of a divider connection in accordance with afirst form of embodiment of the invention;

FIG. 2 shows a first form of embodiment of the leading division scale which it comprises;

FIG. 3 shows thewave shapes of the output signal the circuit in FIG. 2; g

FIG. 4 shows a variant of the division scale;

FIG. 5 shows the connection of the .I inputs of the flip-flops of the circuit in FIG. 4;

FIG. 6 shows a first form of embodiment of the counter-divider comprised 'in the connection in FIG. 1;

FIGS. 7 and 8 show two variants of this counterdivider;

FIG. 9 is a basic diagram of a'counter-divider in accordance with a second form of embodiment;

FIG. 10 illustrates a particular form of embodiment given as an example;

FIG. 11 is a basic diagram of a counter-divider in accordance with a third form of embodiment'of the invention; and

FIG. 12 illustrates a more particular form of embodiment, designed to provide a division ratio varying between 20 and 29.

FIG. 1 shows a leading divider 1 which receives at E the frequency F pulses whose frequency is to be divided, andv whose output drives a tailing counterdivider 2 which delivers the divided frequency at its output S.

Dividers 1 and 2 are of the type known as division scale (scale-of-two with a ratio 2, etc.).

The counter 2 has a capacity N and is set, as soon as it reaches a count n, to send an information to a control input C of the divider 1, i

The divider 1 is set sd as to have a division ratio (M) so long as this information is not enabled, and to pass to the capacity (M+l') when this information is. en

abled.

Moreover, it is not essential for the switching information to be enabled during the N-n last counts of each cycle of the counter 2. To obtain the required result, it is sufficient for said information to be enabled, during each cycle, for a certain number (N-n) of counts of the counter 2 and disabled for the it other counts of the cycle, these two series of counts being able to be interleaved in any way.

In fact, during the Nn counts,-each output pulse of the divider 1 will be generated by M input pulses, while during the n counts, each output pulse of the divider. 1 will be generated by (M+l) input pulses. Finally, the N counts of each cycle of the counter 2, at the end of which it will be returned to zero and will supply at pulse at its output S, will correspond to: M+l )n M (N-n) MN n input pulses of the connection.

nection will itself be programmed.

For example, it is possible to take M= 3, N= 10 and n variable from to 9, which gives a ratio variable from 30 to 39.

In this example, the counter 2 works on a-frequency. 3 or 4 times weaker than the input frequency, and does not comprise any return-to-zero device for a variable capacity. The result is that it can be made very simply and inexpensively, without thereby limiting the connections working frequency; for the latter to depend only on the divider 1, it is sufficient that the coincidence device between the count of the counter 2 and the instruction n should, in the example in question, be capable of switching at least at the frequency F/3, which is easy to achieve. The connections limit working frequency is then completely independent of n.

FIGJZ shows apreferred formof embodiment of the divider l, in the case where M 3.

It includes three flip-flops 4-5-6 of the J K type, whose clock inputs C are driven in synchronism by the input signal E. The flip-flops 4 and 6 have their K inputs connected to a one logical level, while the K input of the flip-flop 5 is connected to the 6 output of the flipflop 6. The Q output of the flip-flop 5 is connected to the J input of flip-flop 4, while the Q output of the flipflop 4 is connected on the one hand to the input of the counter 2 and on the other hand to the J inputs of the flip-flops 5 and 6.

The latter is moreover of the type comprising two inputs J and J driving an internal AND gate 6a. The J input is connected to the switching terminal C26 of the counter 2.

FIG. 3 shows, at (a), (b) and (0) respectively, the wave'shapes of the Q outputs of the flip-flops 4, 5 and 6, when the level is applied at J (which corresponds to the n first counts of the counter'2) and at (d), (e), (f), the same outputs when the one level is applied at J (which corresponds to the (N-n) last counts of the counter 2). The input signal has been shown at (E).

It is obvious that the wave front descending from the first clock pulse (E) makes, in bothcases, flip-flop 4 alone rise (the J input of the two others being at zero).

As soon as it has risen, it makes the J input of the flipflop-S pass to this One level, and, in the second case (J 1) the J input of the flip-flop 6 aswell.

In the first case (J' 0) the J input of the flip-flop 6 remains at zerofThe result is that when the wavefront descends from the next clock pulse and the flipflop 4 is de-actuated, the flip-flop 5 will be actuated,

and the flip-flop 6 remain de-actuated. The actuating of the flip-flop 5 makes its Q output,

and hense the J input of the flip-flop 4, go to zero. The

flip-flop 6 not being actuated, its 0 output, and thus the K input of the flip-flop 5, is at One level. The result is that when the wavefront descends from the next clock pulse, the flip-flop 5 will be de-actuated. The J input of the flip-flop 4 will then go from One level, so that at the next clock pulse, the flip-flop 4 will again be actuated.

At the third pulse, flip-flops 4 and 5 will therefore not change their condition, while flip-flop 6 will be deactuated, which makes the K input of flip-flop 5 go to one.

whose frequency is one-quarter of the clock'frequency.

In other words, with 3 flip-flops'one does get M 3. To obtain M 4, it would be sufficient to add an intermediate flip-flop set and connected like flip-flop 5; for

M 6, two intermediate flip-flops would be needed,

etc.

It should be noted that the informations on the J inputs of the top flip-flops 5-6 are taken into account by the leading flip-flop 4, and therefore by the connection just before the wavefront descending from the 0 signal of the flip-flop 4. For the resolution time of the connection to depend inthe end only onthe response time of flip-flop 4, it istherefore sufficient for the J inputs of the other flip-flops to be able to be positioned during the period of the signal (a). In particular, this counter 2 and its circuit for comparing count and instruction must simply work at the maximum at frequency F/M. A very cheap counter 2, suitable to work e.g. at 12.5 MHz, thus makes it possible to achieve a divider which can work in the given example M= 3 at F= 37.5 MHz.

FIG. 4 shows a divider making it possible to programme the division ratio, either from '20 to 29 or from 30 to 39 or from 40 to 49 or from 50 to 59, according to the position of the switches 12, 13, 14.

The leading counter is made up of five flip-flops 7 to l 1, connected as in FIG. 2 as regards their terminals C, K, O and, in the case of flip-flop 7, terminals J and Q. Like flip-flop 6 in FIG. 2, flip-flop 11 .has two terminals J and J, connected to an internal AND gate (not shown) while flip-flops 8, 9 and 10 each have four terminals J1, J'l, J2, J2, connected to an internal logical unit which has been shown separately in FIG. 5. .T his logical unit includes two AND gates 10a, 10b and an OR gate 10c to form the J K flip-flop 18, whose C input I is connected to the terminal and whose Q output is connected to the switching terminal C26 of the divider 1. An AND gate 19 enables the J input only when the coincidence at 0 is present, and its input connected to the output of 17 comprises a logical inverter element, without thecoincidence at n being so. In this Way, when n O, the J and K inputs of the flip-flop are prevented from being enabled simultaneously, which would make it deliver a toothed waveform at the clock frequency.

In fact, this flip-flop is required to deliver a signal enabled for the n first pulses of each cycle received by the circuit 15 and disabled for the N-n last pulses.

This is indeed the case since, when there is coincidence at n, the K input is'enabled, which de-actuates the flip-flop. At the end of the cycle, the J input is enabled, which reactuates the flip-flop.

' In the second form of embodiment (FIG. 7), the element 20 is a binary amplitude comparator, which compares the binary instruction number n of the corresponding binary number with the count of the counting .At the fourth'pulse, flip-flop5 will therefore be decircuit 15. According to whether the former is greater than, equal to or less than the latter, the output terminal a or b or c is enabled. A device symbolised by the block 3 enables either output a or output b or output c or the logical sum of outputs b and c to be connected to terminal C26.

In the first case, so long as the circuit has not reached the count n, the terminal C26 is enabled, so that the divider has a ratio equal to MN n, as explained above.

In the second case, it is for the N-n last pulses of the cycle that the terminal C26 is enabled.

The division ratio is thus Mn+(M+l )N-n (M+l )N-n.

In the third case, it is for the N-n-l last pulses of the cycle that the terminal C26 is enabled, whence a'division ratio of (n+1) M+ (N-nl)(N+ l)== (M+ l) Nnl Finally, we see that this connection enables a direct spectrum and two inverted spectra to be obtained with the same instruction, which is useful in some applications. An inverted spectrum could also have been obtained with the conne ction in FIG. 6 by connecting the terminal C26 to the Q output of the flip-flop 18.

The connection in FIG. 8 does not make it possible to obtain an inverted spectrum. On the other hand, it simply includes a counting circuit 15 of a particular type, known as a bit rate multiplier (or more commonly by the English abbreviation B.R.M.).

Such a circuit has a counting capacity N and whean an n instruction is applied to it, it delivers atan output (connected here to C26) n output pulses per counting cycle. The result is that the divider 1 will be enabled at the rate of M l n times per cycle, whence the ratio (M+ l)n+M(Nn)=MN+n.

Other forms of embodiment of the dividers l and 2 may be devised by a technician, without departing from the spirit of the invention. The values of M, N and n may be any whatsoever. It is even possible, providing the forms of embodiment in FIGS. 6 or 8 are used, to make n vary between 0 and 10, which will give a division ratio going e.g. from 30 to 40 (or, more generally, from 10 M to l0 (M+l), known counter-dividers do not enable this result to be obtained.

The device described with reference to FIGS. 1 to 7 allows a division ratio in the form MN n or (M+l )N n or (M+l )N n l to be obtained.

In general, N 10 and n programmable from 0 to 9 will be taken.

M will then be the tens digit of the number which expresses the division ratio. If, e.g., M 4, the division ratio will be programmable from 40 to 49 or from 50 to 41 or from 49 to 40.

. In some applications, it would be useful to have available a divider with a ratio programmable e.g. from 43 v to 52 or, more generally, in the form MN n +p, i.e. from any digit of the units.

For this purpose it is sufficient to add to a connection whose second scale is in accordance with FIG. 7, a second binary amplitude comparator comparing the count of the counter with the instruction number p 'and a logical circuit which supplies a supplementary control signal suitable to shift the division ratio of the first scale by two units. 7

FIG. 9 again shows the elements already described above, bearing the same reference numbers, i.e.:

compares a binary instruction number p with the binary number corresponding to the count of the counter 15.

a logical circuit 22, made so as to enable its output 22a, its output 22b, or its output 22c according to the condition of the respective outputs 20a and 21a of the comparators.

More specifically, 22a will be enabled when'20a and 21a are simultaneously enabled: 22b will be enabled I when only one of the two outputs 22a or 22b is enabled.

The leading divider l is set so that its division ratio is equal to M+2, M+l or M, according to whether the control input 22a, 22b or 220 is enabled. For a specific value of p, so long as n is programmed at a value lower than p, the'working of the connection is as follows:

the counter l5.counts the pulses coming from the divider 1. During the n first counts, the count of 15 is less than n and p, so that the outputs 20a and 21a are simultaneously enabled and the division ratio of I is equal to During the p-n ensuing counts, the count of 1 5 is less than p but greater than n, so that only the output 21a is enabled andthe division ratio of l is equal to n+1.

During the .Nn(pn)=N-p last counts, the count outputs 20a and 21a is enabled: the division ratio of l is then equal to n. The result is, finally, that during a comple cycle of the counter 15, for which a pulse is picked up at its output S, the number of pulses applied to the input E was: (M+2)n (M+l) (p-n =MN+p+n.

It is obvious, since p and n play a symmetrical part in this formula, that the result is the same when n21 p.

For example, for' N==l0 (which is the case in the nonlimiting example described), the counter 15 was decimal and having a single decade M=4 and p=3, we get a division ratio variable from 43 to .52 by programming n from O to 9.

The same result would, of course, be obtained by taking n=3 and programming p from 0 to 9.

Making the circuits is within the capacity of a techniclan.

In particular, as we shall see below, the divider 1 can easily be made by means of shift registers of known type. FIG. 12 shows a diagram of an example of a simple embodiment corresponding to M=3 and p=3.

The binary comparator 2 in FIG. 2 can then be made in the form of three NAND gates 211- 212-213, connected to the weight outputs l, 2 and 4 of the counter 15 as the diagram shows. It is obvious that the output of gate 213 is at One level when at least one of its two inputs is at zero level. Now, the output of gate 211 is at zero level when the weight outputs l and 4 of the counter 15 are enabled, i.e. for'count 5; the output of gate 211 is at zero level when'the weight outputs 2 and 4 of counter 15 are enabled, i.e. for count 6 and the two outputs of gates 21 1 and 212 are simultaneously at zero level when the weight outputs l, 2 and 4 of counter 15 are enabled, i.e. for count 7.

During three conditions out of of counter 15, the output of gate 213 is thus enabled.

The logical circuit 22 in FIG. 9 is made in the form of a NOR gate 221 and a NAND gate 222, connected to the output a of the binary amplitude comparator 20 and to the output of the gate 213 in the manner shown.

The outputs of gates 221 and 222 are respectively connected to the parallel inputs B1 and A1 of a shift register 1 whose two other parallel inputs Cl and D1 are at Zero level (symbolised by a grounding). These two clock inputs CC are connected to the input terminal E of the connection; the series input SI is put at One level (symbolised by the sign the QD parallel output is on the one Hand connected to the input of the counter and on the other hand applied to the mode control line MC. The other parallel outputs, not connected, are not shown.

The working of such a register is well known.

When the terminal MC is at One level, the informations present at the parallel inputs are transferred to the outputs by the first clock pulse, therefore QD goes to zero. The result is that MC goes.tozero.- The clock pulses then have the effect of transferring to B1 the information present at A1, to C1 the information present at B1, and to QD the'information present at D1. If B1 is enabled, three pulses are needed to make this transfer. If Al is enabled, Bl not being so, four pulses are needed to make this transfer.

Finally, we see that the register 1 will act as a divider by' 3 when B1 is enabled (irrespective of the condition of Al); as a divider by 4 when Al is enabled, B1 not being so; and as a divider by 5 when neither Al nor B1 is enabled.-

Now, it is obvious that if the outputs of the comparator and of the gate 213 are simultaneously at Zero level, the two inputs A1 and B1 will 'be enabled, whence a 3 division ratio of the register 1. When the output 20a goes to a One value, the output of the gate 213 remaining'at Zero level or vice versa, only the input A1 is enabled, whence a 4 division ratio. When the outputs of the comparator 20 and of the gate 213 are simultaneously enabled, the two inputs A1 and B1 will be at Zero level (whence a 5 division ratio).

The result is that, for n 5, the division ratio of the register will be equal to 4 for the n first counts and for the counts 5-6-7 of the counter 15, while it will be equal to 3 for the counts 8 and 9 and for the 5-n counts following the n first. Whence a division ratio of the connection equal to 4 (n+3) 3 (2+5n) 33+n.

Every time It increases by one unit while still remaining below 8, the division ratio of the register goes from 4to 5 for one of the states 5-6-7 of the counter 15, so that the division ratio of the connection also increases by one unit, thus remaining equal to 33+n.

When n goes from 7 to 8 from 8 to 9, the division ratio of the register goes from 3- to 4 for one of the states S and 9 of the counter 15, so that the division ratio of the connection still remains equal to 33+n.

This ratio therefore varies from 33 to 42 when n is programmed from 0 to 9. To obtain a variation of 33 '8 to 43,,it is sufficient to apply a dummy code to the inputs of 20. 7

It goes without saying that various variants of the connection may be devised by a" technician, without departing from the spirit of the invention.

In particular, the shift register 1 shown in FIG. 9 could be used as a first division scale in any one of the connections illustrated in FIGS. 1 to 8 (for example, this shift register can be the 74-95 type manufactured by the Company TEXAS Instruments lnc."). 7

Moreover, the comparators do not necessarily use the binary code and can moreover be made in the form of a suitable logical circuit.-

In the connections illustrated in FIGS. 1 to 10, the output signal may be very dissymetrical; in fact, as the division ratio of the first scale varies during the second scales counting cycle, the durations of the two halfperiods of the output signal can correspond to two input pulse numbers which are substantially different from each other.

The result is obviously that the extraction of the fundamental component of such a signal poses tricky fil-' tration problems, especially as regards the even harmonics. g

The purposeof the connection illustrated in FIGS. 1 1 and 12 is to eliminate this disadvantage and obtain a strictly symmetrical output signal for the even division ratiosand, for the odd division ratios, an output signal such-thatthe duration of one of the two half-periods does not exceed by more than one step the duration of the other (the step being the duration of an input pulse of the connection).

FIG. 11 shows a first division scale 1 fitted with a pulse input E and a control input C which, when it is enabled, makes the division ratio go from an M 1 value to an M value. The output of this first scale drives a counter 15 fitted with an output S which constitutes the output of the connection. A binary amplitude comparator 20 receives, at its inputs A A and A the respective weight codes 2,-4 and 8 of a binary coded in- I struction number n. The code 1 is applied to an input of an AND gate 23. The output A B of the comparator isconnected to the input C of scale 1.

A zero logical level (which is symbolised by the ground) is permanently applied to the One weight input A of the comparator 20. 1

The counter 15 consists'of a scale-of-five with out puts a, b and c with respective weights 1, 2 and 4,' respectively connected to the inputs 8,, B and B of the comparator 20, and a scale-of-two whose-output; d is connected on the one hand to the output S and on the other hand to the other input ofthe gate 23.

The output of the gate 23 is connected'to a logical circuit 24 with two outputs, respectively connected to the two inputs a B and a B of the comparator 20. The

input a B of the comparator .20 is permanently at Zero level (symbolised by the ground).-

The comparator 20 consits e.g. of the Texas Instruments Inc. integrated circuit type 7485. It is known that, in such a connection, the input have the following decreasing priorities: A B A, B A, B A B and finally a B, a=B or a B. This means that, when A B the output A B is enabled irrespective of the logical levels of the other inputs. Similarly, if A 3;, and A B the output A B is enabled irrespective of the logical levels of the other inputs. For the inputs A B, a B or A B to have an influence on the comparator,

it is necessaryto have A B A B0.

If we consider a counting cycle of the counter 15, during the first counts the output d is at Zero level, while it is at One level during the 5 last counts.

If n is even, the code (1) willnever be enabled. The logical circuit 24 is set, when its input is thus at Zero level, to enable its output a B. On the other hand, if n is odd, the code (1) is enabled, so that the input of the circuit 24 is enabled during the five last counts of the counter 15. The circuit 24 is set so that it then-enables its output a B. It should be noted that the comparator will compare the weight digits 1, 2 and 4 of the count of the counter 15 with the weight digits 2, 4

- and 8 of the instruction n. In other words, it will in fact make a comparison between a binary number A between 1 and 5 supplied by the counter 15 and a binary number also between 1 and' 5, and equal to (n a)/2 with a 0 ifn is even and a =1 ifnis' odd.

During the five last counts of the counter 15, d is at One level, therefore greater than A So long as the count abc is less than (n a)/2, we have: a A,, b A c A and, consequently, the output A B is not enabled. The scale 1 thus divides by M 1. When the 'count abc is equal to -(n a)/2, we get: a A,, b A

c A and d A consequently the output A B is enabled, and the scale 1 divides byM. When the count abc is greater than (n'-- a)/ 2, the output A B is, of

course, also enabled. Finally, the number of pulses at the input E counter by the connection is, during this second half of the cycle, equal to:

During the first half of the cycle, d is at Zero level, therefore equal to A As long as the binary number expressed by the digits abc is less than (n a)/2, the output A B of the comparator is of course enabled, so that the division ratio of the scale 1 is equal to M. When the binary number abc is greater than (n a)/2 the division ratio of the scale 1 is equal to M+l.

When the binary number abc is equal to (N a)/2a A b A 0 A and d ='A In this case, .it is therefore the condition of the inputs a B and a B which determines the enabling of the output A B.

If n is even; the output a B being enabled, A B is too and the division ratio of the scale 1 is M. Consequently, the division ratio of the connection is then equal to: 2M (5-n/2) n (M+ l).

The output signal picked up at S is then strictly symmetrical, since it corresponds to the same numberof input of input pulses for each of the two halves of the cycle. 7

Ifn is odd, the output a B being enabled, A B is not when the number abc is equal to (n a)/2. During the second half of the cycle, the counter 15 therefore then counts one input pulse more than during the first half (the output signal is therefore quasi-symmetrical). In other words, the division ratio isthen: M[5 (n 1/2)] (M +1)(n l)/2+M[5 (n -1/2)1]+(M+l)[(n-l)/2+l) which can also be written:

M [5 -(n-a/2)] (M+l (na)/2 +M [5 (n+a/2)] -F M+1 n+a /2 It will be noted that this latter'formula also covers the case where n is even, since then n 0. v

10 For the counter 15, a divider byN could be used followed by a scale-of-two and the formula giving the division ratio of the connection would then become:

M [N (rt-a/2)] (M+] )(n-a)/2 M [N (n+a/2)] (M+ l )(n-i-a)/2 As an example, for M =2, the division ratio will vary from 20 to 29 for n varying from 0 to 9.

FIG. 12 shosw a simplified connection designed to obtain this particular result.

It can be seen that the code 1 of the number n is then applied directly to the input a B of the comparator 20 and, after inversion by a logical inverter 25, to the input a B. The scale 1 is made in the form of a shift register with a series input SI at One level, its clock inputs CC connected to each other to constitute the input E of the connection, its parallel inputs A1, B1 at One level, its parallel input D1 at Zero level, and its parallel input Cl connected to the enabling terminal A B.

The parallel output QD is connected on the one hand to the input of the counter 15 and on the other hand to the mode control input MC of the register.

It is known that in such a register, when MC is at One level, the informations at the inputs A1 D1 are respectively transferred over the outputs QA QD to the first clock pulse. This first pulse therefore gives Q, Q; l and Q ='O. If A B is enabled, it moreover gives Q 1. As Q, 0, MC goes to Zero level. At that moment, the informations QA Q5 and QC are trans ferred to O by shifting. In other words, at the second clock pulse Q goes to One level. The register thus works on a scale-of-two.

If A B is not enabled, there will be, after the'first pulse, Q 0. It is therefore only at the third pulse that the information 1 present at O will be transferred to O thus bringing the register back to its initial state. It will therefore work, in this case, on a scale-of-three.

As regards the second scale (l5 20 25), its working is the same as that of the scale (15 20 23 24) in FIG. 11. In effect, with n even, the inverter 25 en ables the input B of the comparator. 20, while with n'odd, it enables the input L B.

It goes without saying that various modifications maybe made to the connections described and shown, without departing from the spirit of the invention.

I claim: I

1. Frequency divider comprising a first and a second division scales each having one pulse input and one pulse output, the pulse output of thesecond scale being. I

connected to the pulse output of the first scale, wherein the first scale has a control input and is set so that its division ratio shifts in whole units under the action of control signals applied to its control-input, the second scale including a fixed-capacity counter set to count permanently and to transmit to the first scale control input a control signal and means for enabling the control signal for a predetermined number of the counts displayed by the second scale.

2. Frequency divider in accordance with claim 1, characterised in that the second scale includes a decimal counter and a binary amplitude comparator which compares the count of said counter with said predetermined number.

3. Frequency divider in accordance with claim 1, characterised in that the second scale includes a bit rate multiplier.

4. Frequency divider in accordance with claim 1, characterized in that the second scale moreover incharacterised in that at least one cludes means for comparing the count of the fixedcapacity counter with at least one instruction number to enable at least one control signal and thus shift the division ratio by at least one unit. i

5. Frequency divider in accordance with claim 4, of the instruction numbers is variable.

6. Frequency divider in accordance with claim 4,

- characterised in that, the counter being decimal, a first division ratio shifts in whole units under the action of control signals applied to its control input, the second scale including a fixed-capacity counter set to count permanently-and to transmit to the first scale control input a control signal and means for enabling the control signal for 'a predetermined number of the counts displayed by the second scale, the first scale including a number of J K flip-flops at least equal to the value which its division ratio assumes when the control signal is not enabled, the said flip-flops having clock impulse driven in synchronism by the frequency to be divided, the said flip-flops further having .I and K inputs and Q and Q outputs, the Q output of the first flip-flop driving the J inputs of the other flip-flops andthe pulse input of the second scale, the 6 output of each of said other flip-flops driving the K input of the next flip-flop down with the exception of the first, the Q output of the second flip-flop driving the J input of the first, the K inputs of the first and last flip-flops being connected to a One logical level, the last flip-flop having a further input J to which said control signal is applied.

to the K input of the flip-flop and on the onther hand, via a logical inverter, to a first input of an AND gate having first and second inputs, said second output being directly connected to the second input of the AND gate, said AND gate having an output connected at the J input of the said flip-flop.

12. Frequency devider comprising a first and a second division scales each having one pulse input and one pulse output, the pulse output of the second scale being connected to the pulse output of the first scale, wherein the first scale has a control input and is set so that its division ratio shifts in whole units under the action of control signals applied to its control input, the second scale including a fixed-capacity counter set to count permanently and to transmit to the first scale control input a control signal and meansfor enabling the control signal for a predetermined number of the counts displayed by the second scale, the first division scale comprising a shift register of the type having a mode control input which, according to whether or not it is enabled, makes it possible to obtain the transfer in parallel of information from the parallel inputs to the-corresponding parallel outputs thereof, or to obtain the gradual transfer of the information from one parallel output thereof to the following ones, one of the parallel outputs of said shift register being connected to the fixed-capacity counter and to the said mode control input and means for comparing the count of the fixedcapacity counter with at least one instruction number to enable at least one control signal and thus shift the division ratio by at least one unit, the said comparing means being connected to some of the parallel inputs.

13. Frequency divider comprising a first and a second division scales each having one pulse input and one pulse output, the pulse output of the second scale being 9. Frequency divider in accordance with claim 8,

division ratio shifts in whole units under the action of control signals applied to its control input, the second scale including a fixed-capacity counter set to count permanently and to transmit to the first scale control input a control signal and means for enabling the conconnected to the pulse output of the first scale, wherein the first scale has a control input and is set so that its division ratio shifts by one unit under the action of control signals applied to its control input, the second scale including a fixed-capacity counter set to count permanently and to transmit to the first scale control input a control signal, the second scalefurther including a binary amplitude comparator comparing the count of said fixed-capacity counter with an instruction number .to generate said control signals, the fixed-capacity trol signal for a predetermined number of the counts displayed by the second scale,-the second scale including a decimal counter having first and second outputs, means for generating a One logical level respectively at said first output when the count of-the decimal counter coincides with said predetermined number and at a second output when the said count coincides with the zero value, and a J K flip-flop supplying said control signal and having K and J inputs connected to said first and second outputs respectively.

11. Frequency divider in accordance with claim 10, wherein said first output is connected on the one hand counter comprising a scale-of-N followed by a scale-oftwo, and the binary amplitude comparator'being set to compare the binary number expressed by the logical levels of the parallel outputs of the scale-of-N with a binary number n a/2, n being the instruction number and a a nil value when n is even, and a value equal to 1 when is odd, and means, associated with the comparator, for enabling or not enabling the output signal of the comparator, according to the parity of n, said means only acting for N of the counts of the fixedcapacity counter.

14. Frequency divider in accordance with claim 13, characterised by a comparator comprising K couples of inputs with respectively decreasing priorities, the (K 2) first inputs on one side of the comparator receiving logical levels which define the respective binary digits of the instruction number, with the exception of the binary units digit, while the next input receives a predetermined logical level, the (K 1) first inputs of the other side of the comparator being connected to the respective outputs of the fixed-capacity counter the last couple of inputs of the comparator being connected to means for enabling or not enabling, according to whether the instruction number is even or odd, those of the inputs of said couple which will enable the output signal of the comparator for N of the counts of the fixed-capacity counter.

15. Frequency-divider in accordance with claim 14, characterised by a decimal counter comprising a scaleof-five followed by a scale-of-two,'and by a binary comparator having a first couple of inputs which respectively receive the 1 weight output of the counter and the 8 code of the instruction number, a second couple of inputs which respectively receive the 2 weight output of the counter and the 4code of the instruction number, a third couple of inputs which respectively receive the 1 weight output of the counter and the 2 code of the instruction number, a fourth couple of inputs which respectively receive the 5 weight output of the counter and a Zero logical level, and a fifth couple of inputs respectively connected to the 1 code of the instruction number, directly and via a logical inverter, said couples of inputs having priorities decreasing in the order in which they are enumerated.

* k Iii l 

1. Frequency divider comprising a first and a second division scales each having one pulse input and one pulse output, the pulse output of the second scale being connected to the pulse output of the first scale, wherein the first scale has a control input and is set so that its division ratio shifts in whole units under the action of control signals applied to its control input, the second scale including a fixed-capacity counter set to count permanently and to transmit to the first scale control input a control signal and means for enabling the control signal for a predetermined number of the counts displayed by the second scale.
 2. Frequency divider in accordance with claim 1, characterised in that the second scale includes a decimal counter and a binary amplitude comparator which compares the count of said counter with said predetermined number.
 3. Frequency divider in accordance with claim 1, characterised in that the second scale includes a bit rate multiplier.
 4. Frequency divider in accordance with claim 1, characterized in that the second scale moreover includes means for comparing the count of the fixed-capacity counter with at least one instruction number to enable at least one control signal and thus shift the division ratio by at least one unit.
 5. Frequency divider in accordance with claim 4, characterised in that at least one of the instruction numbers is variable.
 6. Frequency divider in accordance with claim 4, characterised in that, the counter being decimal, a first instruction number varies between 0 and 10n -1, a second instruction number being fixed and less than 10n -1.
 7. Counter-divider in accordance with claim 4, wherein said means are binary amplitude comparators associated with a logical circuit.
 8. Frequency divider comprising a first and a second division scales each having one pulse input and one pulse output, the pulse output of the second scale being connected to the pulse output of the first scale, wherein the first scale has a control input and is set so that its division ratio shifts in whole units under the action of control signals applied to its control input, the second scale including a fixed-capacity counter set to count permanently and to transmit to the first scale control input a control signal and means for enabling the control signal for a predetermined number of the counts displayed by the second scale, the first scale including a number of J K flip-flops at least equal to the value which its division raTio assumes when the control signal is not enabled, the said flip-flops having clock impulse driven in synchronism by the frequency to be divided, the said flip-flops further having J and K inputs and Q and Q outputs, the Q output of the first flip-flop driving the J inputs of the other flip-flops and the pulse input of the second scale, the Q output of each of said other flip-flops driving the K input of the next flip-flop down with the exception of the first, the Q output of the second flip-flop driving the J input of the first, the K inputs of the first and last flip-flops being connected to a One logical level, the last flip-flop having a further input Jx to which said control signal is applied.
 9. Frequency divider in accordance with claim 8, characterised in that the first scale comprises means for varying its division ratio by grounding some of the inputs of the flip-flops which it comprises.
 10. Frequency divider comprising a first and a second division scales each having one pulse input and one pulse output, the pulse output of the second scale being connected to the pulse output of the first scale, wherein the first scale has a control input and is set so that its division ratio shifts in whole units under the action of control signals applied to its control input, the second scale including a fixed-capacity counter set to count permanently and to transmit to the first scale control input a control signal and means for enabling the control signal for a predetermined number of the counts displayed by the second scale, the second scale including a decimal counter having first and second outputs, means for generating a One logical level respectively at said first output when the count of the decimal counter coincides with said predetermined number and at a second output when the said count coincides with the zero value, and a J K flip-flop supplying said control signal and having K and J inputs connected to said first and second outputs respectively.
 11. Frequency divider in accordance with claim 10, wherein said first output is connected on the one hand to the K input of the flip-flop and on the onther hand, via a logical inverter, to a first input of an AND gate having first and second inputs, said second output being directly connected to the second input of the AND gate, said AND gate having an output connected at the J input of the said flip-flop.
 12. Frequency devider comprising a first and a second division scales each having one pulse input and one pulse output, the pulse output of the second scale being connected to the pulse output of the first scale, wherein the first scale has a control input and is set so that its division ratio shifts in whole units under the action of control signals applied to its control input, the second scale including a fixed-capacity counter set to count permanently and to transmit to the first scale control input a control signal and means for enabling the control signal for a predetermined number of the counts displayed by the second scale, the first division scale comprising a shift register of the type having a mode control input which, according to whether or not it is enabled, makes it possible to obtain the transfer in parallel of information from the parallel inputs to the corresponding parallel outputs thereof, or to obtain the gradual transfer of the information from one parallel output thereof to the following ones, one of the parallel outputs of said shift register being connected to the fixed-capacity counter and to the said mode control input and means for comparing the count of the fixed-capacity counter with at least one instruction number to enable at least one control signal and thus shift the division ratio by at least one unit, the said comparing means being connected to some of the parallel inputs.
 13. Frequency divider comprising a first and a second division scales each having one pulse input and one pulse output, the pulse output of thE second scale being connected to the pulse output of the first scale, wherein the first scale has a control input and is set so that its division ratio shifts by one unit under the action of control signals applied to its control input, the second scale including a fixed-capacity counter set to count permanently and to transmit to the first scale control input a control signal, the second scale further including a binary amplitude comparator comparing the count of said fixed-capacity counter with an instruction number to generate said control signals, the fixed-capacity counter comprising a scale-of-N followed by a scale-of-two, and the binary amplitude comparator being set to compare the binary number expressed by the logical levels of the parallel outputs of the scale-of-N with a binary number n - a/2, n being the instruction number and ''''a'''' a nil value when n is even, and a value equal to 1 when is odd, and means, associated with the comparator, for enabling or not enabling the output signal of the comparator, according to the parity of n, said means only acting for N of the counts of the fixed-capacity counter.
 14. Frequency divider in accordance with claim 13, characterised by a comparator comprising K couples of inputs with respectively decreasing priorities, the (K - 2) first inputs on one side of the comparator receiving logical levels which define the respective binary digits of the instruction number, with the exception of the binary units digit, while the next input receives a predetermined logical level, the (K - 1) first inputs of the other side of the comparator being connected to the respective outputs of the fixed-capacity counter, the last couple of inputs of the comparator being connected to means for enabling or not enabling, according to whether the instruction number is even or odd, those of the inputs of said couple which will enable the output signal of the comparator for N of the counts of the fixed-capacity counter.
 15. Frequency-divider in accordance with claim 14, characterised by a decimal counter comprising a scale-of-five followed by a scale-of-two, and by a binary comparator having a first couple of inputs which respectively receive the 1 weight output of the counter and the 8 code of the instruction number, a second couple of inputs which respectively receive the 2 weight output of the counter and the 4 code of the instruction number, a third couple of inputs which respectively receive the 1 weight output of the counter and the 2 code of the instruction number, a fourth couple of inputs which respectively receive the 5 weight output of the counter and a Zero logical level, and a fifth couple of inputs respectively connected to the 1 code of the instruction number, directly and via a logical inverter, said couples of inputs having priorities decreasing in the order in which they are enumerated. 